The present invention relates to a type of steam distributor known as a "steambox" which is used in the manufacture of sheet materials, and in particular, to a steambox design that increases the production rate of paper sheet while decreasing energy utilization. Even more particularly, the present invention relates to a steambox design which substantially eliminates undesirable dripping of condensation onto the paper sheet from the steambox itself and from adjacent structures.
One of the parameters used in grading sheet materials is the moisture content of the material. For example, in the paper production process, various grades of paper having different moisture contents are produced to suit various applications.
Paper production begins with a wet mass of fibers and typically involves several drying processes, the first of which includes impinging the paper sheet material with steam from a steambox and may also include drawing the steam through the sheet with a vacuum box to improve heat transfer from the steam to the sheet. This steam treatment, for example, may cause an increase in sheet temperature of approximately 30.degree. C. The increased temperature decreases the viscosity of water in the sheet. At a later point in the papermaking process, water is squeezed and/or suctioned out of the paper sheet in a section of the papermaking machinery known as the "press section". Because the steam heats the water, the water viscosity is decreased, and thus the pressing and suctioning of water out of the sheet is rendered more effective. In still later drying steps, the sheet is typically passed over several heated steel drums, in the so-called "drying section" of the papermaking machine, to further reduce the moisture content of the sheet. The resulting increase in the dryness of the sheet as it leaves the press section permits an increase in the sheet production rate, as drier paper can move more rapidly through the subsequent drying section.
In the production of many paper products, it is desirable to automatically control the "cross-directional" (i.e., the direction across the width of the sheet perpendicular to the direction of sheet movement) moisture content of the paper sheet using a steambox. Many papermaking machines have scanning moisture sensors which continuously scan back and forth across the width of the sheet and sense the sheet moisture content at various locations across the sheet as the paper is manufactured. The information from this continuous moisture measurement can be fed into a controlling computer. The computer then controls the amount of steam applied by the steambox to various locations across the width of the sheet based upon the sensed moisture content.
Examples of steam distributors are shown in U.S. Pat. Nos. 4,253,247 and 4,580,355, which are incorporated herein by reference. These patents teach a multi-chambered steam distributor in which steam flows from a steam pipe through a valve associated with each chamber, into each chamber, and then is directed to the section of the sheet adjacent to each chamber. The steam flow out of each chamber and toward an adjacent sheet section is controlled by progressively opening or closing the associated valve.
FIG. 1 illustrates an example of a known process in which the present invention may be applied. In particular, FIG. 1 shows a papermaking machine including a steambox 10 to assist in drying the sheet 8. The papermaking machine shown is of the Fourdrinier type and includes a head box 2 feeding a pulp and water mixture 4 to a porous conveyer belt called a "wire" 6 through which the water is drained from the pulp. The paper sheet 8 travels under the steambox 10 and over a vacuum box 12, which, as previously mentioned, assists in drawing the steam through the sheet 8. FIG. 1 also shows an alternate position "A" where the steambox 10 may also be located.
After passing under the steambox 10, the sheet then passes through the press section 14, including pressure rollers 15 and absorbent material 17, further dryers (not shown) and a known scanning moisture sensor 16 which, as previously mentioned, measures the moisture content of the sheet at various locations across the width of the sheet 8. The amount of steam jetted from the steambox 10 at various cross-directional locations is adjusted manually or automatically to reduce the moisture variations in the cross-direction of the sheet 8. As more steam is jetted from a particular steambox chamber onto an opposing section of the sheet, that sheet section becomes hotter and the water viscosity in that sheet section decreases; therefore, the more effectively water may be removed from the hot sheet section. In other words, the use of more steam results in a drier sheet section.
A common problem encountered in altering the moisture content of the sheet via steam treatment is that excess steam that has not been absorbed by the sheet may condense on cool surfaces of the adjacent structures of the paper processing machinery and then drip onto the sheet. The sheet thus moistened will then bear an unsightly water mark. Moreover, this wet portion is weaker than the rest of the sheet and is therefore unusually subject to tearing.
Another shortcoming of the prior art is the use of diffusion plates, which generally contain a large number of small openings through which steam is jetted from the steambox onto the paper sheet. When steam is ejected through these numerous openings, especially numerous small openings, the large quantity of surface area of the steam entrains a relatively large volume of ambient air, which cools the steam before it contacts the sheet; thus, the sheet is not heated as efficiently or as much, and condensation is more likely to occur.
In addition, to the best of applicant's knowledge, the prior art fails to address or remedy the problem of condensation formation within the steambox itself; this condensate could easily drip out of the openings in the diffusion plate and onto the sheet. Such an occurence is particularly troublesome when the machinery is first started up and the steambox is cold.
To overcome the shortcomings of the prior art, it will be appreciated that it is desirable to apply steam to a paper sheet in a manner that will increase the temperature of the sheet while producing a resultant decrease in the viscosity of the water contained in said sheet; this combination of increased heat and decreased viscosity will greatly facilitate immediate and subsequent removal of water from the sheet via a pressing and/or a suctioning process. It will also be appreciated that a steam treatment method that reduces the amount of condensation on the equipment in close proximity to the sheet and that eliminates dripping of water from inside the steambox will reduce the possibility of tearing the sheet, and will also reduce unsightly water marks.